Many known electronic devices utilise embedded micro-controller units (MCUs). Embedded micro-controllers are highly complex devices and typically use a significant amount of current in controlling the operation and functionality of the highly complex device that they are used in. In particular, it is known that a surge of activity is produced upon start-up of the device, which generates a large current transient. This, in turn, has been known to cause the MCU to operate outside of its desired specification.
An example of a large current transient of the MCU, created upon ‘turn-on’ of the electronic device, is illustrated in the waveform 100 of FIG. 1. The waveform 100 illustrates current 105 versus time 110. The sequence of events depicted represents initial power up, running of configuration (‘boot loader’) code including selection of final operating frequency, establishing clocks at the appropriate frequency and finally operation of the MCU using the aforementioned clocks. These events are described in detail below.
Initially, the MCU current increases 115 due to the PLL-locking and the boot loader running, as known to those skilled in the art. Typically a microcontroller is able to use a multitude of crystal oscillators, with a limitation of having only one maximum frequency. Therefore the multiplier required to create the maximum frequency from the crystal frequency is not fixed for the MCU. When power is first applied, the MCU will begin operation using a conservative multiplier, and thus at a conservative frequency. This is shown as 115 in FIG. 1.
Typically one of the tasks of the boot loader is to set the desired multiplier to achieve a maximum MCU operating frequency. Once a corresponding boot-loader operation has begun 115 (as known to those skilled in the art in initialising operation of processors and controllers), thereby creating an initial power surge, a short reduction in the current requirements of the micro controller unit (MCU) occurs.
This occurs during a ‘reset’ stage, whilst the MCU establishes the new multiplier, and hence the new operating frequency. The device multiplying the crystal frequency up to the maximum operating frequency—typically a Phase Lock Loop (PLL)—might overshoot the target maximum frequency. To avoid over speeding the MCU, the known solution is to place the MCU in a reset state, such that any over frequency clock does not cause adverse operation of the MCU. Typically, during this reset state 120, little current is drawn.
Thereafter, the waveform 100 illustrates a rapid increase in current drawn by the MCU up to its full operating current requirements 130 within a rise time of around one clock cycle 125, commonly a few nanoseconds. This occurs when reset is released and the MCU begins operation at a frequency, which may be many times that during the boot loader phase.
Thus, in order to accommodate for this potential for a rapid change in supply current to the MCU, designers typically ‘over-design’ the circuitry by incorporating a more expensive and increased performance power supply circuit, thereby ensuring the power supply remains within its stipulated specification during the MCU ‘turn-on’ operation. Although such a power supply is required to turn on the MCU, it is not required during maximum frequency operation. Therefore, the power supply is over designed for operating the MCU.
In particular, in over-designing the power supply circuit, the voltage regulator used to supply power to the MCU is commonly by nature slower than the MCU, such that it has insufficient time to catch up with the sudden increase in current demanded by the load. Therefore, this additional current typically emanates from the use of expensive and space consuming decoupling capacitors.
Thus, in recent years, a technique known as ‘SoftStart’ has been used to minimise such rapid changes in supply current.
A number of soft start procedures have been developed in the past, such as:                (i) A circuit defined by Clive Bolton in Electronic Design (ED Online ID #2481), which employs a programmable logic device (PLD) to control the start up of a regulator, by use of pulse width modulation (PWM);        (ii) US Patent Application US 20030156438 A1, which defines a soft-start circuit for a regulated power supply, implemented in a feedback circuit with a gradual start-up;        (iii) US Patent Application US 20020140463 A1, which also uses soft-start control for a power supply circuit, breaking the ramping of the output voltage into a series of discrete voltage ramp-up steps;        (iv) US Patent Application describes another power supply circuit implementing a gradual start-up, having a MCU using two operational amplifiers, used to limit the duty cycle of the power supply; and        (v) US Patent Application US 20030020442 A1 documents a current limiting technique for a switching power converter. This circuit slowly increases the switching duty cycle on power up.        
Notably, the power source, such as a voltage regulator, dictates the power to be applied to the load in the aforementioned techniques. This is inappropriate for an MCU, because an MCU only operates within a specified and relatively narrow voltage range. Thus, gradual application of neither voltage nor current will permit the MCU to arrive at its maximum frequency of operation without transient effects, since in both cases the voltage will be too low for MCU operation during the majority of the voltage or current ramp.
Thus, a need has arisen to provide a method of initialising an electronic device, such as an application specific integrated circuit or a micro-controller, wherein the aforementioned disadvantages of known devices and methods for controlling current may at least be alleviated.